Detecting the advance of the moisture front in a bed of sorbent by determining the change in the moisture content of the sorbent as a function of a capacitor in which the sorbent is the dialectric is known in the art and described in U.S. Pat. No. 2,703,628 to Pompeo et al. and in U.S. Pat. No. 4,552,570, patented Nov. 12, 1985 to Gravatt, the latter being commonly assigned and incorporated herein by reference.
Typical commercial driers in use today employ two desiccant chambers, each approximately six feet long, ten inches in diameter, and weighing about six hundred pounds. Capacitance probes used in these large drier chambers generally have taken the form of a series of alternately connected circular discs, about two inches in diameter and 1/16 inch thick. In order to accommodate a sufficient volume of sorbent between each disc to detect the change in dielectric of the sorbent as the moisture level within the sorbent bed increases, the discs are spaced 1/2 inch apart, such that a capacitor with ten discs has a generally cylindrical shape about six inches long and two inches in diameter. This unit is placed transversely in the desiccant bed with respect to the gas flow to detect the moisture front.
Unlike large, commercial driers, adsorbent fractionators for some applications such as those disclosed in U.S. application Ser. No. 736,479 filed May 21, 1985 as well as such specialized applications as avionics cooling, macro and micro-personnel cooling, and purifying air for breathing in fixed wing and rotary wing aircraft, watercraft, hospitals, and the like often necessitate stringent weight and size requirements. For example, in order to meet size specifications for certain aircraft applications, the desiccant beds may need to be as short as about 0.4 to 0.6 feet long and have anywhere between four and sixteen inch diameter chambers, resulting in a small chamber that, even filled with adsorbent particles, weighs only about fifty pounds.
It has been appreciated that the capacitor design described above is not a mechanically acceptable design for this configuration of bed. This is because the capacitor should not only sample a relatively thin section of desiccant to accurately sense the location of the moisture front but should also sample a large fraction of the cross section of the bed to assure a representative reading. An improved design for short, yet relatively large diameter bed adsorption systems is necessary.